The work rolls of rolling mills are rotationally driven by an electric motor. The rotationally driving power is transmitted from the electric motor to the pair of upper and lower work rolls through a transmission having a power distributing function and through a pair of spindles.
The type of spindles used varies depending on how its use, such as rolling conditions. For example, UJ spindles (Universal Joint, also called propeller shafts) are used in rolling mills for rolling materials with normal hardness, and gear spindles are used in hard material rolling mills for rolling materials with relatively high hardness (e.g. a material with a 40%-reduction deformation resistance value of 70 [kg/mm2]).
High tensile strength steels which are even harder materials (also called high tensile materials, e.g. a material with a 40%-reduction deformation resistance value of 130 [kg/mm2]) have been developed for the purpose of making structures such as automobiles stronger and lighter. Thus, there have been needs for high tensile strength steel rolling mills for rolling high tensile strength steels, and there have accordingly been needs for high performance gear spindles for use in high tensile strength steel rolling mills. These high performance gear spindles usable in high tensile strength steel rolling mills are required to satisfy conditions (1), (2), and (3) described below.
(1) Small Diameter
High tensile strength steel rolling mills use work rolls with smaller diameters than normal cases so as to suppress increase in rolling force. The work rolls are formed of a pair of upper and lower rolls, and are independently coupled to their own gear spindles and rotationally driven when rolling power (rotational power) is transmitted thereto through the gear spindles. Thus, the gear spindles are formed of a pair of upper and lower gear spindles like the work rolls. A portion of the gear spindles coupled, to their work rolls needs to have a diameter smaller than the work rolls' diameter so that the gear spindles installed in a pair on the upper and lower sides will not interfere with each other.
For example, in a hard material rolling mill, a work roil diameter DW is 330 [mm], and a gear-spindle outer diameter D is 325 [mm]. On the other hand, in a high tensile strength steel rolling mill, the work roll diameter DW is required to be 250 [mm] to limit the rolling force, and the gear-spindle outer diameter D is required to be 245 [mm] to prevent the vertical interference between the gear spindles.
Here, the work roll diameter DW is the minimum usable diameter. As the work rolls are used for rolling, their surfaces become worn due to the contact with the rolling target strips, and the surfaces are often polished with a polishing machine. Accordingly, the work roll diameter DW gradually becomes smaller with use. The difference between the largest diameter and the smallest diameter of work rolls is generally around 10% approximately.
(2) Ability to Transmit Larger Torque
Rolling torque T of a rolling mill is influenced by a deformation resistance value F of the rolling target strip and the work roll diameter DW and is therefore such that T∝f(F)+f(DW). As mentioned earlier, the work roll diameters DW of high tensile strength steel rolling mills are smaller than the work roll diameters DW of hard material rolling mills, and the deformation resistance values of high tensile strength steels are significantly larger than the deformation resistance values of conventional hard materials. Thus, rolling torque required for the rolling of high tensile strength steels is larger than rolling torque required for the rolling of conventional hard materials.
For example, a strength index T/D3 of allowable transmission torque Ta of each gear spindle in a conventional hard material rolling mill (T: necessary transmission torque per gear spindle [ton·m], D: gear-spindle outer diameter [mm]) is such that T/D3≦0.4 [ton/m2], while the strength index T/D3 of the allowable transmission torque Ta of each gear spindle in a high tensile strength steel rolling mill is such that T/D3≈0.6 to 0.8 [ton/m2]. As described above, as the deformation resistance value of the rolling target strip increases, the strength index T/D3 of the allowable transmission torque Ta increases as well. (The strength index T/D3 [ton/m2] of the allowable transmission torque Ta is an expression omitting “×109.” To be precise, the expression is (T/D3)×103 [ton/m2] because the gear-spindle outer diameter D [mm] is plugged in after unit conversion into a gear-spindle outer diameter D×10−3 [m]. Hereinbelow, in this description, the strength index of the transmission torque T with respect to the gear-spindle outer diameter D [mm] will be expressed as T/D3 [ton/m2] in the same short form as above.)
(3) Ability to Rotate at High Speed
The production performance of a rolling mill is expressed by a multiplier of the strip thickness, the strip width, and the rolling speed. In general, rolled products are produced from rolling target strips with a fixed strip thickness and strip width, and the production performance of a rolling mill is dependent on its rolling speed. Rolling speed V of a rolling mill is influenced by the work roll diameter DW and work roll rotational speed N, and is therefore such that V∝DW×N. As mentioned earlier, the work roll diameters DW of high tensile strength steel rolling mills are smaller than the work roll diameters DW of hard material rolling mills. Thus, with the same rotational speed N, the rolling speeds V of the high tensile strength steel rolling mills are inevitably lower, and the production performance of the rolling mills is lower as well. Then, in order to ensure the same production performance as the hard material rolling mills, the high tensile strength steel rolling mills need to rotate their work rolls at higher speeds than the hard material rolling mills. In other words, the high tensile strength steel rolling mills require gear spindles capable of high speed rotation.
For example, in the case of a conventional hard material rolling mill including work rolls with a work roll diameter DW of 330 [mm], its gear spindles are required to have specifications which can handle a rotational speed of 1930 [rpm] to achieve production performance equivalent to a rolling speed of 2000 [mpm]. However, in a case of a high tensile strength steel rolling mill with a smaller work roll diameter DW of 250 [mm], its gear spindles are required to have high speed rotation specifications which can handle a rotational speed of 2546 [rpm], which is approximately 1.3 times larger than the rotational speed in the conventional case, to achieve the same production performance as above equivalent to a rolling speed of 2000 [mpm].
It is generally known that as rotation of a rotary body becomes faster, the rotary body may exhibit flexural vibration, rattling vibration, torsional vibration, or the like, and resonance or the like may greatly influence the rotary body. In the case of a rolling mill, its gear spindles, which are rotary bodies, may easily break due to the resonance. Even if the resonance does not occur, the vibration is still transmitted to the rolling target strip and results in uneven strip thickness, a poor strip shape, and poor surface texture, which can cause a significant quality loss to the rolling target strip. For this reason, gear spindles capable of high speed rotation need to be resistant to vibration. Specifically, gear spindles which are light and short and in which gaps with less backlash are required.
For example, a resonance rotational speed He indicative of the likelihood of vibration of a rotary body such as a gear spindle is influenced by the outer diameter D of the rotary body and the length L of the rotary body and is therefore such that Nc∝(D)/f(L). In other words, the smaller the outer diameter of the rotary body, the more likely that resonance will occur, and the longer the rotary body, the more likely that resonance will occur.
The functions required for gear spindles for high tensile strength steel rolling described above have a problem of interfering with other functions.
The allowable transmission torque Ta of a gear spindle is dependent on an inclination angle θ at which the gear spindle is used and on the gear-spindle outer diameter D. The smaller she inclination angle θ and the larger the gear-spindle outer diameter D, the larger the allowable transmission torque Ta of the gear spindle. Thus, the allowable transmission torque Ta is such that Ta∝f(D)/f(θ). The gear spindle is rotated with one side coupled to a transmission and the other side coupled to a work roll. Thus, if the height of the axis of the work roll and the height of the axis of the transmission are the same, the inclination angle θ of the gear spindle is 0°, which is the optimal condition in view of strength.
In the case of a high tensile strength steel rolling mill, however, the work roll diameter must be smaller than normal cases so as to limit the rolling load, while the size of the transmission must be increased for transmitting high torque. Hence, the difference between the height of the axis of the transmission and the height of the axis of the work roll is larger than conventional cases. Accordingly, the inclination angle θ of a gear spindle for high tensile strength steel rolling is inevitably larger than those of normal gear spindles, meaning that the allowable transmission torque of the gear spindle is less than normal cases.
To avoid this, that is, to make the gear spindle inclination angle θ equal to or smaller than normal cases under the condition where a difference ΔH between the heights of the two axes is large, the gear spindle length L needs to be increased to reduce the increase in the gear spindle inclination angle θ caused by the increase in the difference ΔH between the heights of the two axes, as can be figured out from tan θ=ΔH/L. However, there is the problem in that a gear spindle for high tensile strength steel rolling is more likely to vibrate due to its small outer diameter, and also the vibration is even more likely to occur if the gear spindle length L is increased. Thus, reducing the inclination angle θ is difficult co achieve with current techniques.
Meanwhile, it is assumed that in the future, ultra-high tensile strength steels harder than high tensile strength steels will be developed and that rolling mills for rolling the ultra-high tensile strength steels, and higher performance gear spindles capable of handling such rolling will be required. In other words, for the conditions (1) , (2), and (3) described above, it is necessary to realize an even smaller diameter, greater allowable transmission torque, and faster rotation than conventional cases.